Conditioned pain modulation is associated with common polymorphisms in the serotonin transporter gene

Treating Chronic Pain with SSRIs: What Do We Know?

Serotonin is a monoamine neurotransmitter that plays a major role in both nociception and mood regulation. Alterations in the 5-hydroxytryptophan (5HT) system have been reported in chronic pain patients. In recent years, Selective Serotonin Reuptake Inhibitors (SSRIs) have been suggested as an alternative treatment for chronic pain due to the fact that they are better tolerated presenting less secondary effects than other antidepressants such as tricyclic antidepressants. Although several clinical trials have been published, the effectiveness of SSRI as treatment for pain conditions is inconclusive. This review aims to summarise what is known, regarding the effectiveness of SSRI as a treatment for chronic pain conditions in adults. A total of 36 studies involving a total of 1898 participants were included in this review. Of the 36 trials included in the review, 2 used zimelidine as treatment, 3 used escitalopram, 4 used fluvoxamine, 4 used sertraline, 6 used citalopram, 8 used paroxetine, 9 used fluoxetine, and one used both citalopram and paroxetine. Because the trials included in this review are quite heterogeneous, only qualitative analyses were performed. SSRI seems to have an effect on most of chronic pain conditions; however, further clinical trials with good methodology leading to low risk of bias are needed in order to conclude once and for all the effect of this drug class as treatment for chronic pain conditions.

Pain, Genes, and Function in the Post-Hip Fracture Period

Post-hip fracture generalized pain can lead to a progressive decline in function and greater disability. The purpose of this study was to explore the factors that influence pain among older adults post-hip fracture, including genetic variability, and evaluate whether pain directly or indirectly influenced upper and lower extremity function. This was a secondary data analysis using data from the first 200 participants in a Baltimore Hip Study (BHS), BHS-7. Assessments were done at 2 months post-hip fracture and included age, sex, marital status, education, cognitive status, comorbidities, body mass index (BMI), upper and lower extremity function, single nucleotide polymorphisms (SNPs) from 10 candidate genes, and total areas of pain and pain intensity. Model testing was done using the AMOS statistical program. The full sample included 172 participants with an average age of 81. Fifty percent were female and the majority was Caucasian (93%). Model testing was done on 144 individuals who completed 2 month surveys. Across all models, age, cognition, and BMI were significantly associated with total areas of pain. Thirty SNPs from five genes (BDNF, FKBP5, NTRK2, NTRK3, and OXTR) were associated with areas of pain and/or pain intensity. Together, age, cognition, BMI, and the SNP from one of the five genes explained 25% of total areas of pain and 15% of pain intensity. Only age and cognition were significantly associated with lower extremity function, and only cognition was significantly associated with upper extremity function. The full model was partially supported in this study. Our genetic findings related to pain expand prior reports related to BDNF and NTRK2.

Is the experience of thermal pain genetics dependent?

It is suggested that genetic variations explain a significant portion of the variability in pain perception; therefore, increased understanding of pain-related genetic influences may identify new targets for therapies and treatments. The relative contribution of the different genes to the variance in clinical and experimental pain responses remains unknown. It is suggested that the genetic contributions to pain perception vary across pain modalities. For example, it has been suggested that more than 60% of the variance in cold pressor responses can be explained by genetic factors; in comparison, only 26% of the variance in heat pain responses is explained by these variations. Thus, the selection of pain model might markedly influence the magnitude of the association between the pain phenotype and genetic variability. Thermal pain sensation is complex with multiple molecular and cellular mechanisms operating alone and in combination within the peripheral and central nervous system. It is thus highly probable that the thermal pain experience is affected by genetic variants in one or more of the pathways involved in the thermal pain signaling. This review aims to present and discuss some of the genetic variations that have previously been associated with different experimental thermal pain models.

No association of polymorphisms in the serotonin transporter gene with thermal pain sensation in healthy individuals

Background

Recent studies have suggested an association between genotypes affecting the expression of the serotonin transporter and thermal pain perception and the thermal grill. The aim of this study was to investigate differences in thermal and mechanical pain perception and the thermal grill in two groups of healthy volunteers according to their genotype, associated with either high (n = 40) or low (n = 40) expression of the serotonin transporter and according to gender. Cold and warm detection and pain thresholds, pressure pain threshold and cold, warm and pain sensations to single or alternating stimuli with cold (20°C) and warm (40°C) temperatures (known as the thermal grill) were determined. In addition, intensity of ongoing pain and area and intensity of pinprick hyperalgesia in the secondary hyperalgesic area following topical application of capsaicin and vehicle control (ethanol) were determined.

Results

No significant differences in detection and pain thresholds for cold and warm temperatures, presence of paradoxical heat sensation, pressure pain threshold and pain responses to suprathreshold thermal stimuli were observed. There was also no difference in capsaicin-evoked ongoing pain and secondary hyperalgesia between the two genotype groups (p >0.4), also when subdivided by gender (p >0.17). In addition, there were no significant differences in the perception of the thermal grill between the two genotypes (p >0.5), also when subdivided by gender.

Conclusions

Genotypes associated with high or low expression of the serotonin transporter were not associated with thermal pain thresholds, pressure pain threshold, pain after capsaicin application or responses to the thermal grill.

The present results do not support that the investigated genotypes play a major role in thermal pain perception among healthy individuals.

Cerebral responses and role of the prefrontal cortex in conditioned pain modulation: an fMRI study in healthy subjects

The mechanisms underlying conditioned pain modulation (CPM) are multifaceted. We searched for a link between individual differences in prefrontal cortex activity during multi-trial heterotopic noxious cold conditioning and modulation of the cerebral response to phasic heat pain. In 24 healthy female subjects, we conditioned laser heat stimuli to the left hand by applying alternatively ice-cold or lukewarm compresses to the right foot. We compared pain ratings with cerebral fMRI BOLD responses. We also analyzed the relation between CPM and BOLD changes produced by the heterotopic cold conditioning itself, as well as the impact of anxiety and habituation of cold-pain ratings. Specific cerebral activation was identified in precuneus and left posterior insula/SII, respectively, during early and sustained phases of cold application. During cold conditioning, laser pain decreased (n=7), increased (n=10) or stayed unchanged (n=7). At the individual level, the psychophysical effect was directly proportional to the cold-induced modulation of the laser-induced BOLD response in left posterior insula/SII. The latter correlated with the BOLD response recorded 80s earlier during the initial 10-s phase of cold application in anterior cingulate, orbitofrontal and lateral prefrontal cortices. High anxiety and habituation of cold pain were associated with greater laser heat-induced pain during heterotopic cold stimulation. The habituation was also linked to the early cold-induced orbitofrontal responses. We conclude that individual differences in conditioned pain modulation are related to different levels of prefrontal cortical activation by the early part of the conditioning stimulus, possibly due to different levels in trait anxiety.

5-HTT SS genotype is associated with the pro-nociceptive sensation by alcoholic sting

The purpose of this study was to examine the relationship between the genetic polymorphism in the promoter of the SLC6A4 gene encoding the serotonin transporter (5-HTT) and the sensitivity to noxious stimulation from a clinical perspective. The genotyping of the 217 outpatients with mild epidermal abrasion in lateral crural region was performed by a combination of polymerase chain reaction and digestion. The intensity of pain to medical alcohol treatment was rated on a visual analog scale (VAS). The results suggest that the human triallelic 5-HTT genotypes are related to individual differences in sensitivity to alcoholic sting. According to the VAS ratings, the subjects with the 5-HTT low-expression genotype reported more pain than those with 5-HTT medium- and high-expression genotypes following test stimuli. There is no significant difference between sexes in the same SLC6A4 genotype and between medium and high expressions of 5-HTT subjects. Taken together, our study supports the hypothesis that the transcription rate of the 5-HTT transporter may play an important role in the pain sensitivity and central sensitization.

Relationship between Personality Traits and Endogenous Analgesia: The Role of Harm Avoidance

BACKGROUND

Whether psychological factors such as anxiety and pain catastrophizing levels influence the expression of endogenous analgesia in general and, more specifically, the conditioned pain modulation (CPM) response is still under debate. It may be assumed that other psychological characteristics also play a role in the CPM response. The neurotransmitters serotonin, dopamine, and norepinephrine are involved both in CPM, as well as personality traits such as harm avoidance (HA), novelty seeking (NS), and reward dependence (RD), which can be obtained by the Tridimensional Personality Questionnaire (TPQ). However, the associations between these traits (HA, NS, and RD) with endogenous analgesia revealed by CPM have not yet been explored.

METHODS

Healthy middle-age subjects (n = 28) completed the TPQ, Spielberger's State Anxiety Inventory, and the Pain Catastrophizing Scale and were assessed for CPM paradigms using thermal phasic temporal summation as the "test stimulus" and hand immersion into hot water bath (CPM water) or contact heat (CPM contact) for "conditioning stimulus."

RESULTS

Higher levels of HA were associated with less-efficient CPM responses obtained by both paradigms: CPM water (r = 0.418, P = 0.027) and CPM contact (r = 0.374, P = 0.050). However, NS and RD were not associated with the other measurements. No significant relationship was observed between state anxiety and pain catastrophizing levels and the CPM responses.

CONCLUSIONS

The relationship between the capacity of endogenous analgesia and the tendency to avoid aversive experience can be explained by mutual mechanisms involving similar neurotransmitters or brain areas. These findings illuminate the key role of harm avoidance obtained by the TPQ in determining the characteristics of pain modulation profile.

Analysis of meaningful conditioned pain modulation effect in a pain-free adult population

Conditioned pain modulation (CPM) encompasses the effects of inhibitory and facilitatory pain modulatory systems and is inefficient in some chronic pain states. A proportion of healthy subjects also exhibit little or no CPM, perhaps suggesting that inherent factors such as gender or genetics may be influential. However, there is no consensus on how best to determine a meaningful CPM effect. This study aimed to determine the proportion of pain-free subjects exhibiting a meaningful CPM effect. Analyses of associations between 5HTTLPR (serotonin transporter-linked polymorphic region) polymorphisms on the serotonin transporter gene (SLC6A4), gender, and CPM effect were also carried out. A total of 125 healthy subjects (47 male; 78 female) underwent pressure pain threshold testing before, during, and after a cold pressor conditioning stimulus. A buccal cell sample was collected for analysis of 5HTTLPR genotype. Meaningful CPM effect was determined as an increase in pressure pain threshold values from baseline greater than the inherent error of measurement, calculated as 5.3%. During the conditioning stimulus, 116 subjects (92.8%) exhibited a CPM effect whereas 9 did not. CPM effect did not differ significantly between genders or between 5HTTLPR genotypes. This provides a clear basis on which to determine the proportion of patients with a chronic pain disorder that exhibit a meaningful CPM effect.

PERSPECTIVE

This study proposes a method for calculating meaningful CPM effect and reports the proportion and magnitude of effect elicited in a large sample. Associations between CPM, gender, and genotype were also analyzed. Clarification of normal CPM response may help to elucidate the mechanisms driving CPM inefficiency in chronic pain.

Serotonergic modulation of pain and analgesic responses: a study in rats with constitutionally altered serotonin transporters

BACKGROUND

A role of the serotonin (5HT) transporter, a key regulator of serotonergic transmission, in the physiology, pharmacology and genetics of pain responses has been proposed recently. The present study aimed to explore the impact of constitutive differences in the activity of the serotonin transporter, and 5HT homeostasis in general, on the modulation on pain sensitivity and analgesic responses to drugs that utilize 5HT mechanisms.

METHODS

A novel genetic animal model, Wistar-Zagreb 5HT rats, obtained by selective breeding of animals for extreme activity of the platelet serotonin transporter was used. As a consequence of breeding, two sublines of this model, termed high-5HT and low-5HT, differ in both central and peripheral serotonin homeostasis. Thermal pain sensitivity of 5HT sublines was assessed at baseline and following administration of analgesic drugs, as determined by paw withdrawal latency to radiant heat stimulation.

RESULTS

Animals from 5HT sublines show differences in both basal pain sensitivity and analgesic responses. Rats with the low-5HT phenotype displayed decreased baseline paw withdrawal latencies (hyperalgesia) in comparison to their high-5HT counterpart (25%; p < 0.001). They also showed better analgesic response to acute and prolonged treatment with tramadol (p = 0.027) and clomipramine (p = 0.019), respectively, whereas administration of fluvoxamine did not produce an analgesic effect in either 5HT subline.

CONCLUSIONS

These findings support the idea that functionality of the serotonin transporter is one of the physiological/genetic determinants of individual differences in pain responses and modulation. They also validate Wistar-Zagreb 5HT rats, with constitutionally up-regulated/down-regulated serotonin transporter, as a potential new genetic model for studying serotonergic modulation of pain responses.

The serotonin transporter gene polymorphism is associated with the susceptibility and the pain severity in idiopathic trigeminal neuralgia patients

BACKGROUND

To investigate the possible association between the serotonin transporter gene (5-HTTLPR) and rs 25531 polymorphism and the susceptibility and the pain severity in Trigeminal Neuralgia patients.

METHODS

A total of 244 TN patients and 280 age and sex matched healthy volunteer were recruited. 5-HTTLPR and rs 25531 genotyping were performed. All patients received the carbamazepine treatment and the treatment response was evaluated at 6 months.

RESULTS

The genotype distribution of 5-HTTLPR between TN patients and controls were significantly different. The TN Patients had a higher prevalence of short-short genotype than controls. The short-short genotype carriers are also significantly associated with higher pain severity and poorer carbamazepine treatment response compared to the long-long genotype carriers. In contrast, the rs 25531 polymorphism was not associated with the susceptibility to TN, neither with the pain severity and the treat response to carbamazepine.

CONCLUSION

The 5-HTTLPR polymorphism is associated with the susceptibility to TN and pain severity of TN.

A large conditioned pain modulation response is not related to a large blood pressure response: a study in healthy men

BACKGROUND

Endogenous pain modulation has been studied with the conditioned pain modulation (CPM) paradigm with large differences in the magnitude of the CPM effect. We hypothesized that differences in CPM effects might be associated with differences in blood pressure responses to the conditioning stimulus when comparing the CPM effects using two different conditioning stimuli.

METHODS

A single-blind repeated-measures design with block-randomization was applied on 25 healthy male subjects. The test stimulus (TS; tonic heat pain for 120 s) was first presented alone, thereafter in parallel with a conditioning stimulus (CS). Conditioning stimuli were either a cold pressor test (CPT) or equally painful ischaemic muscle pain (ISC), both lasting 120 s. Finger blood pressure and heart rate were recorded continuously. Data were analysed in a linear mixed model framework with CS type (CPT or ISC) and conditioning (TS or TS + CS) as independent factors.

RESULTS

An inhibitory CPM effect was found for both types of conditioning (p < 0.001). The CPM effect was larger during CPT conditioning compared with ISC conditioning (p = 0.001). No association with the concomitant cardiovascular response (blood pressure and heart rate) was found (p > 0.34).

CONCLUSION

Cold pressor pain CS induces larger CPM effects than ischaemic pain CS. The larger CPM effect is, however, not associated with a larger blood pressure response. Other factors related to the CS should be investigated to understand why different CS modalities give different CPM effects.

Predicting fibromyalgia, a narrative review: are we better than fools and children?

Fibromyalgia syndrome (FMS) is a common and intriguing condition, manifest by chronic pain and fatigue. Although the pathogenesis of FMS is not yet completely understood, predicting the future development of FMS and chronic pain is a major challenge with great potential advantages, both from an individual as well as an epidemiological standpoint. Current knowledge indicates a genetic underpinning for FMS, and as increasing data are accumulated regarding the genetics involved, the prospect of utilizing these data for prediction becomes ever more attractive. The co-existence of FMS with multiple other functional disorders indicates that the clinical identification of such symptom constellations in a patient can alert the physician to the future development of FMS. Hypermobility syndrome is another clinical (as well as genetic) phenotype that has emerged as a risk factor for the development of FMS. Stressful events, including early life trauma, are also harbingers of the future development of FMS. Functional neuroimaging may help to elucidate the neural processes involved in central sensitization, and may ultimately also evolve into markers of predictive value. Last but not least, obesity and disturbed sleep are clinical (inter-related) features relevant for this spectrum. Future efforts will aim at integrating genetic, clinical and physiological data in the prediction of FMS and chronic pain.

Polymorphism in serotonin receptor 3B is associated with pain catastrophizing

Pain catastrophizing, a coping style characterized by excessively negative thoughts and emotions in relation to pain, is one of the psychological factors that most markedly predicts variability in the perception of pain; however, only little is known about the underlying neurobiology. The aim of this study was to test for associations between psychological variables, such as pain catastrophizing, anxiety and depression, and selected polymorphisms in genes related to monoaminergic neurotransmission, in particular serotonin pathway genes. Three hundred seventy-nine healthy participants completed a set of psychological questionnaires: the Pain Catastrophizing Scale (PCS), the State-Trait Anxiety Inventory and Beck’s Depression Inventory, and were genotyped for 15 single nucleotide polymorphisms (SNPs) in nine genes. The SNP rs1176744 located in the serotonin receptor 3_B gene (5-HTR3B) was found to be associated with pain catastrophizing scores: both the global score and the subscales of magnification and helplessness. This is the first study to show an association between 5-HTR3B and PCS scores, thus suggesting a role of the serotonin pathway in pain catastrophizing. Since 5-HTR3B has previously been associated with descending pain modulation pathways, future studies will be of great interest to elucidate the molecular pathways involved in the relation between serotonin, its receptors and pain catastrophizing.

How genes and the social environment moderate each other

Recent research has suggested that the social environment can moderate the expression of genetic influences on health and that genetic influences can shape an individual's sensitivity to the social environment. Evidence supports 4 major mechanisms: genes can influence an individual's response to environmental stress, genes may enhance an individual's sensitivity to both favorable and adverse environments, inherited characteristics may better fit with some environments than with others, and inherited capabilities may only become manifest in challenging or responsive environments. Further progress depends on better recognition of patterns of gene-environment interaction, improved methods of assessing the environment and its impact on genetic mechanisms, the use of appropriately designed laboratory studies, identification of heritable differences in an individual before environmental moderation occurs, and clarification of the timing of the impact of social and genetic moderation.

Associations between serotonin transporter gene polymorphisms and heat pain perception in adults with chronic pain

BACKGROUND

The triallelic serotonin transporter gene linked polymorphic region (5-HTTLPR) has been associated with alterations in thermal pain perception. The primary aim of this study was to investigate the associations between heat pain (HP) perception and the triallelic 5-HTTLPR in a large cohort of adults with chronic pain.

METHODS

The cohort included 277 adults with chronic pain who met inclusion criteria, and were consecutively admitted to an outpatient pain rehabilitation program from March 2009 through March 2010. Individuals were genotyped for the triallelic 5-HTTLPR (including rs25531) and categorized as high, intermediate, or low expressors of the serotonin transporter. Standardized measures of HP perception were obtained using a validated quantitative sensory test method of levels.

RESULTS

The distribution of the high, intermediate, and low expressing genotypes was 61 (22%), 149 (54%) and 67 (24%), respectively. The Hardy-Weinberg P-value was 0.204 which indicated no departure from equilibrium. A significant effect of genotype was observed for values of HP threshold (P = 0.029). Individual group comparisons showed that values of HP threshold were significantly greater in the intermediate compared to the high expressing group (P = 0.009) but not the low expressing group (P > 0.1). In a multiple variable linear regression model, the intermediate group (P = 0.034) and male sex (P = 0.021) were associated with significantly greater values of HP 0.5, but no significant genotype-by-sex interaction effect was observed.

CONCLUSIONS

In this study that involved adults with chronic pain, the intermediate triallelic 5-HTTLPR expressing group, but not the low expressing group, was associated with greater HP thresholds compared to the high expressing group.

Placebo effects in children: a review

Of more than 155,000 PubMed citations found with the search term "placebo," only ~9,000 (5.8%) included the terms "children" or "adolescents." When all these papers were screened, only ~2,000 of them investigated the placebo effect per se, and of those, only ~50 (2.5%) discussed the placebo effect in children and adolescents. In this narrative review, we explore four aspects of the placebo response in children and adolescents: (i) the legal and ethical limitations and restrictions for the inclusion of children in clinical trials as well as in experimental (placebo) research that may explain the poor knowledge base; (ii) the question of whether or not the placebo effect is larger in children and adolescents as compared with adults; (iii) whether the mechanisms underlying the placebo effect are similar between children and adults; and (iv) whether mediators and moderators of the placebo effect are comparable between children and adults. We finally discuss some of the consequences from the current placebo research in adults that may affect both experimental and clinical research in children and adolescents.

Pharmacogenetics of chronic pain and its treatment

This paper reviews the impact of genetic variability of drug metabolizing enzymes, transporters, receptors, and pathways involved in chronic pain perception on the efficacy and safety of analgesics and other drugs used for chronic pain treatment. Several candidate genes have been identified in the literature, while there is usually only limited clinical evidence substantiating for the penetration of the testing for these candidate biomarkers into the clinical practice. Further, the pain-perception regulation and modulation are still not fully understood, and thus more complex knowledge of genetic and epigenetic background for analgesia will be needed prior to the clinical use of the candidate genetic biomarkers.

Nature and nurture of human pain

Humans are very different when it comes to pain. Some get painful piercings and tattoos; others can not stand even a flu shot. Interindividual variability is one of the main characteristics of human pain on every level including the processing of nociceptive impulses at the periphery, modification of pain signal in the central nervous system, perception of pain, and response to analgesic strategies. As for many other complex behaviors, the sources of this variability come from both nurture (environment) and nature (genes). Here, I will discuss how these factors contribute to human pain separately and via interplay and how epigenetic mechanisms add to the complexity of their effects.

The role of stimulation parameters on the conditioned pain modulation response

Background and purpose

Conditioned pain modulation (CPM) is a testing paradigm representing features of diffuse noxious inhibitory control. There is large diversity in the paradigms applied to induce CPM, and the consistency in CPM responses assessed by different paradigms is largely unknown. We aimed to characterize and explore the associations between the CPM responses assessed by different paradigms in the same cohort.

Methods

Thirty-three healthy middle-aged subjects underwent six CPM paradigms. The ‘test-stimuli’, consisted of thermal and mechanical modalities, using pain thresholds, suprathreshold pain and temporal summation types of measurements. The ‘conditioning-stimulus’ consisted of a contact heat stimulus applied to the thener of the left hand for 60s at an intensity of 46.5°C.

Results

Large variability was observed among the responses to the different CPM paradigms. Surprisingly, no correlations were found between the various CPM responses.

Conclusions

The variability in the CPM responses may suggest that the capacity of pain modulation is a multifaceted trait, whose expression varies with the application of different CPM paradigms.

Implications

Considering that CPM responses may represent different processes when assessed by different paradigms, we encourage the use of more than one CPM paradigm.

Personalized pain medicine: the clinical value of psychophysical assessment of pain modulation profile

Experimental pain stimuli can be used to simulate patients' pain experience. We review recent developments in psychophysical pain testing, focusing on the application of the dynamic tests-conditioned pain modulation (CPM) and temporal summation (TS). Typically, patients with clinical pain of various types express either less efficient CPM or enhanced TS, or both. These tests can be used in prediction of incidence of acquiring pain and of its intensity, as well as in assisting the correct choice of analgesic agents for individual patients. This can help to shorten the commonly occurring long and frustrating process of adjusting analgesic agents to the individual patients. We propose that evaluating pain modulation can serve as a step forward in individualizing pain medicine.

Induction of the perceptual correlate of human long-term potentiation (LTP) is associated with the 5-HTT genotype

The purpose of the present study was to examine how genetic variability in the promoter of the SLC6A4 gene encoding the serotonin transporter (5-HTT) may influence induction of long-term potentiation (LTP). The genotyping of the 53 healthy volunteers was performed by a combination of TaqMan assay and gel electrophoresis. Based on the transcription rates, the subjects were divided in 3 groups; 5-HTT SS, 5-HTT SL(G)/L(A)L(G)/SL(A) and 5-HTT L(A)L(A). The intensity of pain to test stimuli was rated on a visual analog scale (VAS). High frequency stimulation (HFS) conditioning applied to one arm was used to induce LTP. Only a minor change in pain was observed following the HFS conditioning evoked by electrical test stimuli delivered through the conditioning electrode. Moreover, the change in pain evoked by test stimuli delivered through the conditioning electrode was not related to the 5-HTT genotype. However, we observed a clear increase in pain following the HFS conditioning evoked by mechanical pin-prick test stimuli delivered at the skin adjacent to the conditioning. Also, the 9 individuals with the 5-HTT SS genotype reported more pain than individuals with 5-HTT SL(G)/L(A)L(G)/SL(A) genotype following HFS conditioning on mechanical pin-prick test stimuli. Thus, the present data show that induction of the perceptual correlate of human LTP is associated with the genetic variability in the gene encoding the 5-HTT. Taken together, this suggests that the expression of 5-HTT, may be important for induction of LTP in humans.

Abnormal endogenous pain modulation is a shared characteristic of many chronic pain conditions

The intensity of acute and chronic pain depends on interactions between peripheral impulse input and CNS pain mechanisms, including facilitation and inhibition. Whereas tonic pain inhibition is a characteristic of most pain-free individuals, pain facilitation can be detected in many chronic pain patients. The capability to inhibit pain is normally distributed along a wide continuum in the general population and can be used to predict chronic pain. Accumulating evidence suggests that endogenous pain inhibition depends on activation of the prefrontal cortex, periaqueductal gray and rostral ventral medulla. Quantitative sensory test paradigms have been designed to acquire detailed information regarding each individual's endogenous pain inhibition and facilitation. Such tests include: temporal summation of pain, which is mostly used to assess facilitatory pain modulation by measuring the change in pain perception during a series of identical nociceptive stimuli; and conditioned pain modulation, which tests pain inhibition by utilizing two simultaneously applied painful stimuli (the 'pain inhibits pain' paradigm). Considerable indirect evidence seems to indicate that not only increased pain facilitation but also ineffective pain inhibition represents a predisposition for chronic pain. This view is supported by the fact that many chronic pain syndromes (e.g., fibromyalgia, temporomandibular joint disorder, irritable bowel syndrome, headache and chronic fatigue syndrome) are associated with hypersensitivity to painful stimuli and reduced endogenous pain inhibition. However, future prospective studies will be necessary to provide definitive evidence for this relationship. Such research would not only provide important information about mechanisms relevant to chronic pain but would also permit identification of individuals at high risk for future chronic pain.

Serotonin-1A receptor polymorphism (rs6295) associated with thermal pain perception

Background

Serotonin (5-HT) is highly involved in pain regulation and serotonin-1A (5-HT1A) receptors are important in determining central 5-HT tone. Accordingly, variation in the 5-HT1A receptor gene (HTR1A) may contribute to inter-individual differences in human pain sensitivity. The minor G-allele of the HTR1A single nucleotide polymorphism (SNP) rs6295 attenuates firing of serotonergic neurons and reduces postsynaptic expression of the receptor. Experiments in rodents suggest that 5-HT1A-agonism modulates pain in opposite directions at mild compared to high noxious intensities. Based upon this and several other similar observations, we hypothesized that G-carriers would exhibit a relative hypoalgesia at mild thermal stimuli but tend towards hyperalgesia at higher noxious intensities.

Methods

Fourty-nine healthy individuals were selectively genotyped for rs6295. Heat- and cold-pain thresholds were assessed along with VAS-ratings of a range of suprathreshold noxious heat intensities (45°C–49°C). Nociceptive-flexion reflex (NFR) thresholds were also assessed.

Results

Volunteers did not deviate significantly from Hardy-Weinberg equilibrium. G-carriers were less sensitive to threshold-level thermal pain. This relative hypoalgesia was abolished at suprathreshold noxious intensities where G-carriers instead increased their ratings of heat-pain significantly more than C-homozygotes. No differences with regard to NFR-thresholds emerged.

Conclusion/Significance

To the best of our knowledge this is the first study of human pain perception on the basis of variation in HTR1A. The results illustrate the importance of including a range of stimulus intensities in assessments of pain sensitivity. In speculation, we propose that an attenuated serotonergic tone may be related to a ‘hypo- to hyperalgesic’ response-pattern. The involved mechanisms could be of clinical interest as variation in pain regulation is known to influence the risk of developing pain pathologies. Further investigations are therefore warranted.

Pharmacogenetics of pain and analgesia

Pain severity ratings and the analgesic dosing requirements of patients with apparently similar pain conditions may differ considerably between individuals. Contributing factors include those of genetic and environmental origin with epigenetic mechanisms that enable dynamic gene-environment interaction, more recently implicated in pain modulation. Insight into genetic factors underpinning inter-patient variability in pain sensitivity has come from rodent heritability studies as well as familial aggregation and twin studies in humans. Indeed, more than 350 candidate pain genes have been identified as potentially contributing to heritable differences in pain sensitivity. A large number of genetic association studies conducted in patients with a variety of clinical pain types or in humans exposed to experimentally induced pain stimuli in the laboratory setting, have examined the impact of single-nucleotide polymorphisms in various target genes on pain sensitivity and/or analgesic dosing requirements. However, the findings of such studies have generally failed to replicate or have been only partially replicated by independent investigators. Deficiencies in study conduct including use of small sample size, inappropriate statistical methods and inadequate attention to the possibility that between-study differences in environmental factors may alter pain phenotypes through epigenetic mechanisms, have been identified as being significant.

Recent advances in the understanding of genetic susceptibility to chronic pain and somatic symptoms

Regional (e.g., low back) and widespread chronic pain disorders are common in the general population and are known to be heritable. Recent research suggests that genetic factors increase the risk of developing chronic pain independent of the site of pain. Candidate gene studies have been conducted on key pathways to elucidate susceptibility genes that are likely to be involved in both the sensory and affective components of pain. Findings have been largely equivocal, predominantly due to small sample size, but larger studies of pain in general population samples are being conducted. Interesting candidate genes from animal models and monogenic pain disorders are beginning to emerge. Recent advances in genetics research have yet to make an impact in the pain field but provide considerable scope for future research efforts.

Genetic basis of pain variability: recent advances

An estimated 15-50% of the population experiences pain at any given time, at great personal and societal cost. Pain is the most common reason patients seek medical attention, and there is a high degree of individual variability in reporting the incidence and severity of symptoms. Research suggests that pain sensitivity and risk for chronic pain are complex heritable traits of polygenic origin. Animal studies and candidate gene testing in humans have provided some progress in understanding the heritability of pain, but the application of the genome-wide association methodology offers a new tool for further elucidating the genetic contributions to normal pain responding and pain in clinical populations. Although the determination of the genetics of pain is still in its infancy, it is clear that a number of genes play a critical role in determining pain sensitivity or susceptibility to chronic pain. This review presents an update of the most recent findings that associate genetic variation with variability in pain and an overview of the candidate genes with the highest translational potential.